The main reason is to stop any degrading of the beer or mask the changes in flavour when it has degraded. Beer degrades in hot weather and sunlight. It's also used to keep flies out of the beer. So when it comes down to it, it's just become a bit of a tradition.
to give stupid people something to think about.
Mexico.
A Mexican will drink beer, water, soda or whatever they are in the mood for.
Beer
tecate a Mexican beer.
Chocolate, root beer, grape, lemon.........
Usually nothing. I have been known to squeeze a few drops of lemon juice into beer, and I have seen people dump in salted peanuts and challenge themselves to drink it without spilling the foam on the floor.
Dos Equis.
Corona
no there is no peanuts in beer put you could put the in if you want
drink beer and eat curry, ...or Mexican
Corona Extra = 4.6